1. Field of the Invention
The present invention relates to apparatus for separating a liquid phase from a liquid-solid mixture, and more particularly, it pertains to a rotary screw type of liquid-solid separating apparatus.
2. Description of the Prior Art
Screw finishers are extensively used in the citrus fruit processing industry to separate fruit juice from the mixture of the juice, pulp and seeds produced by extraction apparatus; to separate frit (fine pulp particles) from a mixture of water, oil and the frit; and to separate particulate pulp matter from citrus pulp wash mixtures. In one type of screw finisher that is now being used in the citrus processing industry, the discharge opening for the solid, or deliquified, phase of the mixture is formed between the relatively large end of the screw of the finisher and a stripper ring which is biased toward the end of the screw by a pneumatic diaphragm valve that is set to maintain a selected pressure thereby maintaining a relatively constant pressure within the finisher under varying flow rates through the finisher. One major problem with this type of screw finisher is that it is not capable of handling a wide range of consistencies of solid-liquid mixures. For example, when it is desired to separate juice from a mixture containing seeds or coarse peel particles, the annular opening between the stripper ring and the end of the screw must be wide enough to permit the seeds or coarse particles to pass therethrough. Thus, the annular opening must be greater than 3/8 in.; however, this opening size at the large diameter of the annular opening necessitates too great a discharge passage to maintain adequate pressure on the mixture so as to adequately express juice through the screen in the finisher. Accordingly, the finisher cannot be used to separate mixtures of liquids and solids that include solids with very coarse particle sizes.
Another problem with the aforementioned type of prior art finisher is that it will not efficiently separate mixtures that are fed at very low flow rates, even when seeds or other coarse particle sizes are not present. In the aforementioned finisher, the stripper ring must be spaced by about 0.004 in. from the rotating screw to prevent wear of these parts. Since the ring is so large, the total annular opening created by the small clearance is so substantial that the finisher will not maintain adequate pressure on mixtures fed at low flow rates to the finisher.
U.S. Pat. No. 742,971 to Cummer issued Nov. 3, 1903, discloses a screw type of apparatus for separating liquids from a solid-liquid mixture that utilizes a perforate discharge chamber wall having a conical shape at its downstream end adapted to receive a screw conveyor having a complementary conically tapered, helical flight thereon. The chamber terminates in a flat end wall and the discharge opening is formed in the side of the chamber at the bottom thereof. A gate is pivotally mounted to the discharge chamber to cover the opening, and the gate is biased into a closed position by an adjustable counterweight so that the gate will resist flow through the opening and thus enhance the pressure of the mixture within the apparatus.